The correspondence between seismic velocity anomalies in the crust and mantle and the differential incision of the continental-scale Colorado River system suggests that signifi cant mantle-to-surface interactions can take place deep within continental interiors. The Colorado Rocky Mountain region exhibits low-seismic-velocity crust and mantle associated with atypically high (and rough) topography, steep normalized river segments, and areas of greatest differential river incision. Thermochronologic and geologic data show that regional exhumation accelerated starting ca. 6-10 Ma, especially in regions underlain by low-velocity mantle. Integration and synthesis of diverse geologic and geophysical data sets support the provocative hypothesis that Neogene mantle convection has driven long-wavelength surface deformation and tilting over the past 10 Ma. Attendant surface uplift on the order of 500-1000 m may account for ~25%-50% of the current elevation of the region, with the rest achieved during Laramide and mid-Tertiary uplift episodes. This hypothesis highlights the importance of continued multidisciplinary tests of the nature and magnitude of surface responses to mantle dynamics in intraplate settings.

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Travel times from first-arriving P (Pg) and S (Sg) waves and P wave Moho reflections (PmP) are inverted for P and S wave seismic velocities. Seismic data were recorded along two lines of instruments oriented N42E (X) and S57E (Y) with lengths of 152 km and 195 km, respectively (Figure 1-main text). The X and Y lines included 937 and 1079 instruments and average station spacings of 160 and 180 m, respectively. Most instruments along these two lines were Reftek 125A (Texan) seismographs with 4.5 Hz geophones. Within 7.5 km of Mount St. Helens, Nodal Seismic instruments with 10 Hz geophones were also incorporated into the data sets. Eight borehole shots were recorded along each line of instruments. The inner four shots were ~450 kg and were approximately 15 and 30 km from Mount St. Helens. The outer shots were ~900 kg and were approximately 50 and 75 km from Mount St. Helens. In addition to the eight shots, an M 2.1 earthquake with a depth of 19.1 km and epicenter near the southwest end of the X line was recorded during the active-source experiment. Travel time data from this event are used in the X line inversion. Record sections with Pg, PmP, and Sg travel time picks from outer and inner shots are shown in Figure DR1. Additional reflections observed in the data are not used in this study. The data are band-pass filtered from 1-16 Hz for the Pg and PmP picks and 1-10 Hz for the Sg picks. The final data set used for the inversions consists of 11,525 Pg

[1] Support for the Colorado high topography is resolved using seismic data from the Colorado Rocky Mountain (CRM) Experiment and Seismic Transects. The average crustal thickness, derived from P wave receiver function imaging, is 48 km. However, a negative correlation between Moho depth and elevation is observed, which negates Airy-Heiskanen isostasy. Shallow Moho (<45 km depth) is found beneath some of the highest elevations, and therefore, the CRM are rootless. Deep Moho (45-51 km) regions indicate structure inherited from the Proterozoic assembly of the continent. Shear wave velocities from surface wave tomography are mapped to density employing empirical velocity-to-density relations in the crust and mantle temperature modeling. Predicted elastic plate flexure and gravity fields derived from the density model agree with observed long-wavelength topography and Bouguer gravity. Therefore, lowdensity crust and mantle are sufficient to support much of the CRM topography. Centers of Oligocene volcanism, e.g., the San Juan Mountains, display reduced crustal thicknesses and lowest average crustal velocities, suggesting that magmatic modification strongly influenced modern lithospheric structure and topography. Mantle velocities span 4.02-4.64 km/s at 73-123 km depth, and peak lateral temperature variations of 600 C are inferred. S wave receiver function imaging suggests that the lithosphereasthenosphere boundary is 100-150 km deep beneath the Colorado Plateau, and 150-200 km deep beneath the High Plains. A region of negative arrivals beneath the CRM above 100 km depth correlates with low mantle velocities and is interpreted as thermally modified/metasomatized lithosphere resulting from Cenozoic volcanism.